Generally, contacts to sources and drains to active devices may be made through an inter-layer dielectric (ILD) formed to help electrically isolate the active devices from overlying metallization layers. These contacts may be made by forming an opening through the ILD in order to expose those portions of the substrate to which the contacts are desired to be formed, and then forming a self-aligned silicide (otherwise known as a salicide). The salicide may be formed by forming a metal layer in the opening and against the substrate and then annealing the metal layer and substrate in order to form the salicide. Excess metal may then be removed, thereby leaving the salicide, and contacts may be formed in connection with the salicide.
To aid in the process of forming the salicide, a pre-clean may be performed on the openings prior to the formation of the metal layer within the openings. This pre-clean has historically been performed using either a directional physical bombardment using, e.g., argon, krypton, or xenon ions. However, directional physical bombardment can damage the opening's profile, leading to such damage as chopping (a disfigurement of the sidewall of the opening along the top of the opening) and bowing (a disfigurement of the sidewalls of the opening along the sidewalls of the opening). Additionally, the directional physical bombardment may cause damage to the bottom of the opening, thereby leading to an uncontrolled salicide formation.
Alternatively, an in-situ chemical pre-clean using, e.g., an etchant may be utilized. However, while the chemical pre-clean can reduce or eliminate some of the drawbacks associated with directional physical bombardment, the chemical pre-clean may also cause the opening to enlarge due to the isotropic etching of the opening. This enlargement of the opening can lead to the opening failing to meet the critical dimensions (CD) that may be desired or required for the contact, and may lead to the isolation of the contact plug to the metal gate.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.